Electromagnetic bus coupling

ABSTRACT

An assembly (for example, an assembly in the form of an interposer that is distinct from a motherboard and from the devices that communicate with the motherboard) includes electromagnetic couplings. Each of the electromagnetic couplings couples signals being communicated between a device and a bus. Each of the electromagnetic couplings is connected to (a) an associated bus connector to provide connections of the couplings to the bus, and (b) an associated device connector to provide connections of the couplings to the device.

BACKGROUND

[0001] This description relates to electromagnetic bus coupling.

[0002] Electromagnetic couplings can be used, for example, to coupledata between electronic devices and a communication bus (e.g., amulti-drop bus) in place of direct electrical connections. Such anarrangement is proposed in U.S. Pat. No. 5,638,402.

DESCRIPTION

[0003]FIG. 1 shows an electromagnetic coupling.

[0004]FIG. 2 is a schematic side view of a multi-drop bus couplingsystem.

[0005]FIG. 3 is a schematic three-dimensional view of a multi-drop buscoupling system.

[0006]FIG. 4 is a top view of sockets and an interposer.

[0007]FIG. 5 is a cross-sectional view of a socket and an interposer.

[0008]FIG. 6 is a cross-sectional view of a socket and a motherboard.

[0009]FIG. 7 is a cross-sectional view of part of a bus coupling system.

[0010] As shown in FIG. 1, in electromagnetic bus coupling, signals arecommunicated between a device 10 and a bus 12 by electromagneticcoupling between two conductors 14, 16 that are close together.

[0011] One way to hold the conductors close together, described here, isby mounting them, for example, on a single structural medium (“buriedcoupling”), for example, in the form of two nearby traces on a motherboard. Conversely, the two conductors may be mounted in two separatestructural media that are then connected to hold the conductors closetogether on two “sides” of a coupling interface (“interface coupling”).

[0012] In one proposed method of interface coupling, the conductor thatis on the device side is formed on a flexible substrate, and theconductor that is on the bus side is formed on an exposed surface of amotherboard. The flexible substrate is pressed against the surface ofthe motherboard and held in place to establish the electromagneticcoupling. In another interface coupling approach, both conductors areheld in a socket into which a daughterboard bearing the device isinserted. The socket is soldered to the motherboard bus traces.

[0013] Buses generally have many lines, and the electromagnetic couplingis provided for each of the lines of the bus, for example, using manyconductors on the flexible substrate or in the socket.

[0014] Buried coupling can be achieved, for example, inside amotherboard that carries the bus or on a daughter card. Buried couplingon the motherboard may be done, for example, laterally between tracesformed on a single plane of the motherboard or vertically between tracesformed on different layers of the motherboard.

[0015] In a typical multi-drop bus, the bus extends across amotherboard, and two or more sockets are arranged at respective droplocations along the bus. At each location, a socket may be mounted onthe motherboard and connected to lines of the bus and to power andground carried in the motherboard. A daughter card may be inserted intothe socket so that a device included on the daughter card maycommunicate with the bus.

[0016] Another technique for achieving electromagnetic coupling isillustrated by the example shown schematically in FIG. 2, in which thecouplings are buried in a discrete integrated structure that may becalled an interposer. At each drop position 18 a . . . 18 c, anelectromagnetic coupling 20 a . . . 20 c is buried on an interposer 22,which, in some examples may be implemented on a multi-layer circuitboard. The interposer 22 is connected to a bus 24, which is formed on amotherboard 26. A socket 30 a . . . 30 c is electrically connected tothe coupling for communication and is attached mechanically to themotherboard 26 for support. A daughter card 32 a . . . 32 c is insertedinto the socket. The daughter card includes a device 34 a . . . 34 cthat communicates through the bus.

[0017] Forming the couplings in an interposer, compared to forming themon a motherboard, for example, has advantages. By burying theelectromagnetic couplings in an interposer, design and manufacturingchoices about the motherboard, the sockets or the daughter cards neednot be altered or compromised. The costs of the motherboard, sockets,and daughter cards may therefore be kept down. Tighter manufacturingtolerances and extremely good coupling performance can be achieved usingthe interposer approach. The interposer may make use of dielectricmaterials and conductor thicknesses and widths that may be smaller thanthose typically used on motherboards, which typically use thickerdielectrics and conductor dimensions to assure mechanical robustness. Inthe interposer, the couplers may be formed of high performancedielectric layers that have desirably high dielectric constants ormaterials whose electromagnetic properties may be varied by applyinglocal static electric or magnetic bias fields, thus permitting thecoupling coefficient of the individual couplers to be varied in order tooptimize the available transmission bandwidth. Such materials typicallywould not be integrated into a motherboard structure for economic andmanufacturing reasons. Well-developed substrate manufacturing technologycan be used to achieve highly uniform coupling performance from unit tounit. Incorporating the transceiver on the interposer reduces the needfor space on the daughter card.

[0018] A wide variety of implementations are possible. One example isshown in FIG. 3.

[0019] In FIG. 3, the interposer 22 is formed as a multi-layer circuitboard that is mounted along the length of the bus 24. Each socket 30 cis mounted on the mother board 26 at one of the drop positions 18 c.(Only one socket is shown in FIG. 3; seven other drop positions areshown schematically.)

[0020] As shown in FIG. 4 (a top view of eight sockets and aninterposer), the interposer may be surface-mounted on the motherboardusing two ball grid arrays (BGA) of solder balls 50, 52 at opposite endsof the interposer. The upper surface of the motherboard and the bottomsurface of the interposer have patterns of contact pads corresponding tothe BGA. The motherboard contact pads for one of the BGAs 50 areconnected to signal and power and ground conductors in the motherboard.The motherboard contact pads for the other of the BGAs 52 are isolatedand provide mechanical support for the interposer. Although not shown inFIG. 4, additional rows of solder balls may be used to attach theinterposer to the motherboard (for mechanical support) in the spaces 56between adjacent sockets.

[0021] At each drop location, a signal processing circuit 60 a, 60 b . .. 60 c is mounted on the top of the interposer circuit board. Also ontop of the interposer at each drop location are holes of a micro pingrid array 62, which are arranged to receive pins of a socket.

[0022] Signal processing circuit 60 may, for example, be a transceiverintegrated circuit (TiC) that is configured to provide a bidirectionallink by modulating signals passing from the daughter card to the bus andby demodulating signals passing from the bus to the daughter card.

[0023] The connection of the interposer to the motherboard bus at onlyone end helps to reduce parasitic effects arising from theinterconnection. The extended bus traces in the interposer areelectrically terminated at the end opposite to the end at which thetraces are connected to the motherboard bus. The use of BGA mountingalso reduces the effect of the connection on signal integrity. The BGApad configurations may also be designed to control impedance effects atthe connections.

[0024] As shown in FIG. 5, each socket 30 a may include an array of pins70 that match the micro pin grid array of the interposer and a well 72that houses the interposer when the socket is mounted. The socket alsoincludes a set of mechanical mounting clips 74. The socket has a slot(not shown) to receive a daughter card. Contact conductors arranged inthe slot make contact with, for example, edge pads arranged on a surfaceof the daughter card. The interposer includes traces that run the lengthof the interposer and carry the bus signals and voltage and ground, ineffect forming an extension of the bus.

[0025] As shown in FIG. 6, each of the clips 74 includes two resilientlegs 80, 82, having tapered ends. The legs lock into place when the clipis forced into a corresponding hole in the motherboard. Clearance 84 isprovided so that the lateral position of the interposer relative to themotherboard may vary without placing lateral stress on the solder ballsof the ball grid arrays or on the pins of the pin grid, which have tighttolerance requirements to assure electrical connection.

[0026] As shown in FIG. 7 (an enlarged side sectional view), signalstravel along the path indicated by the double-ended arrows between thebus and the socket at a given drop position 18 c. The signals passbetween a bus trace 60 on the surface of the motherboard 26 and acorresponding extended bus trace 66 embedded in the interposer 22through the medium of a ball 62 of the ball grid array 50 and aconductive via 64 formed in the interposer 22. The signals pass alongextended bus trace 66, which extends along the length of the interposerto all of the drop positions. A region 67 of the extended bus trace 66at drop position 18 c serves as a conductor for electromagnetic couplingwith a second conductive trace 68 embedded on a different layer of theinterposer. (Trace 68 is coupled at one end to a via 80 and isterminated at the other end by a resistor 69 (through another via.))

[0027] The electromagnetically coupled signal then passes alongconductive via 80 and solder ball 82 into circuit 60 c. After processingby circuitry 84, the signal passes through solder ball 81 and a via 97to a trace 68 a and then to a plated through hole 79, and then into pin70 that carries the signal to the socket 30 c. From there the signalpasses to the daughter card 32 c. (Note that, in the figure, the pin 70does not electrically touch the trace 66 nor does it touch the circuit84 but rather is behind each of them.)

[0028] During assembly, the TiC dice may be directly attached to theinterposer using a C4 flip chip process, or the TiC may be packed inFCBGA packages and then reflowed to the interposer.

[0029] The populated interposer is then attached to the motherboardusing the BGAs. Then each socket is attached electrically by insertingits pins into the interposer micro pin grid array (PGA) plated-throughholes and mechanically by inserting its support clips into thethrough-holes in the motherboard.

[0030] The system described above, and other examples, have one or moreof the following advantages and may also have other advantages. Becausethe electromagnetic coupling and interconnection functions are performedon the interposer, the layout and stackup design of the interposer isnot constrained by other requirements (for example requirements that mayapply if those functions are embedded on a motherboard, as discussedearlier). The lack of constraints enables substantial flexibility forinterposer design and manufacture, permitting achievement of optimalcoupling performance with controlled variation. The electromagneticcoupling on the interposer can easily be made either between traces on acommon plane, or between traces on different planes. The dielectriclayer can be made thin to permit the coupling region to be miniaturizedfor a given coupling strength requirement. High precision manufacturingprocesses may be applied to the interposer to achieve tight tolerances.Coupling characteristics can be achieved with high electricalperformance repeatability and high mechanical reliability and stabilityacross large numbers of manufactured units in high-volume,cost-constrained applications. The amount of space and the pitchrequired for each daughter card on the motherboard may be small.Incorporating the signal processor on the interposer reduces therequirement for placing special circuitry on the daughter card, spacethat is often not available. The TIC chip is integrated to theinterposer, creating a cost effective component without requiringdaughter card modification. Physical limitations of the daughter cardlayout can be disregarded. The effect on signal integrity fromtransferring the main signal path off the motherboard is reduced byutilizing only one surface mount interconnect for multiple drops.

[0031] Although some examples have been described above, otherimplementations are also within the scope of the following claims.

[0032] For example, although the interposer was described as having theextended bus traces running within the board of the interposer, theextended bus traces could be provided on a surface of the interposer orimplemented in other ways. Similarly, although the TiC circuitry isshown being mounted on a surface of the interposer, that circuitry couldbe implemented in other ways, for example mounted on the daughter card.Other techniques of mounting the interposer on the motherboard and ofconnecting the bus to the interposer extended bus traces could be usedin addition to the BGAs described earlier. The sockets need not bemounted by a PGA but could be attached to permanently on the interposerboard by surface mount or other techniques.

1. Apparatus comprising an integrated assembly including electromagneticcouplings, each of the electromagnetic couplings to electromagneticallycouple signals being communicated between a device and a bus, each ofthe electromagnetic couplings being connected to (a) an associated busconnector to provide connections of the couplings to the bus, and (b) anassociated device connector to provide connections of the couplings tothe device.
 2. The apparatus of claim 1 in which the integrated assemblycomprises an interposer, the device comprises a daughter board, and thebus is part of a motherboard.
 3. The apparatus of claim 1 in which eachof the couplings includes conductors between which the electromagneticcoupling occurs, the conductors of each of the couplings being formed infixed positions within the assembly.
 4. The apparatus of claim 1 inwhich the assembly also includes driver circuitry for processing thesignals on the device side of the coupler.
 5. The apparatus of claim 4in which the driver circuitry is embodied in a flip chip.
 6. Theapparatus of claim 1 in which the associated device connector is toprovide electrical connections to a socket.
 7. The apparatus of claim 1in which the bus connector is to provide electrical connections to thebus.
 8. The apparatus of claim 7 in which the assembly is connected tothe bus by only one junction and the bus connectors of the assembly areconnected to the one junction.
 9. Apparatus comprising a socket assemblyincluding conductors to make electrical connection with conductors of acircuit board that is to be mounted on the socket assembly, terminals tomake electrical connection with a second assembly that includeselectromagnetic couplings, and mechanical supports to be attached to amotherboard on which the second assembly is mounted.
 10. The apparatusof claim 9 in which the socket assembly is elongated, the mechanicalsupports are located nearer to opposite ends of the socket assembly andthe terminals are located nearer the middle of the socket assembly. 11.The apparatus of claim 9 in which the mechanical supports compriseresilient lockable pins.
 12. A system comprising a bus, an assemblyconnected to the bus, the assembly including electromagnetic couplingsadapted for communicating signals between the bus and devices served bythe assembly, and sockets connected to the assembly and to receivecircuit boards which contain the devices.
 13. The system of claim 12also including a motherboard on which the bus is formed.
 14. The systemof claim 13 also including daughter cards mounted in the sockets. 15.The system of claim 12 in which each of the electromagnetic couplingsincludes conductors between which the electromagnetic coupling occurs,the conductors of each of the couplings being formed in fixed positionswithin the assembly.
 16. The system of claim 12 in which the assemblyalso includes driver circuitry for processing the signals on the deviceside of the coupler.
 17. The system of claim 16 in which the drivercircuitry is embodied in a flip chip.
 18. The system of claim 12 inwhich the assembly is connected to the bus by only one junction and theelectromagnetic couplings are connected to the one junction. 19.Apparatus comprising an assembly including means for electromagneticcoupling, each of the means being to electromagnetically couple signalsbeing communicated between a device and a bus, each of the means beingconnected to (a) an associated bus connector to carry the signalsbetween the coupling and the bus, and (b) an associated device connectorto carry the signals between the coupling and the device.
 20. Theapparatus of claim 19 in which the assembly also includes means forprocessing the signals on the device side of the coupler.
 21. A methodcomprising receiving signals from a bus on an assembly that is separatefrom a motherboard on which the bus is formed, electromagneticallycoupling at least some of the signals at the assembly, and sending thesignals towards a device.
 22. The method of claim 21 also includingprocessing the signals at the assembly after the signals have beenelectromagnetically coupled.
 23. The method of claim 21 also includingelectromagnetically coupling others of the signals at the assembly, andsending the other signals towards at least one other device. 24.Apparatus comprising a bus, an interposer connected at one junction onthe bus, electromagnetic couplings at separate locations on theinterposer, and conductors to carry signals from the electromagneticcouplings towards respective devices.
 25. The apparatus of claim 24 alsoincluding circuitry to process the signals from the couplings anddeliver then to the conductors.
 26. A method comprising attaching anassembly to a bus of a motherboard, the assembly includingelectromagnetic couplings, and connecting a socket to the assembly, thesocket to receive a circuit board.
 27. The method of claim 26 in whichthe attaching includes soldering a ball grid array.
 28. The method ofclaim 26 in which the connecting includes attaching the socket at amicro pin grid array.
 29. The method of claim 26 in which the connectingincludes attaching the socket to the motherboard.